The present invention relates to an ion trap type mass spectrometer by which wideband auxiliary AC electric fields having the frequency components within the required range are created, then ions having mass-to-charge ratios within the required range are ejected by resonance, and only specific species of ions are analyzed at high sensitivity and high resolution, or only specific species of dissociated ions are analyzed using the tandem mass (MS/MS) method.
The corresponding ion trap type mass spectrometer, as shown in FIG. 2, consists of ring electrode 10 and end cap electrodes 11 and 12 arranged vertically facing one another so as for the ring electrode to be located in between. Hereinafter, the ring electrode and the end cap electrodes are collectively referred to as the ion trap electrodes.
Quadruple-pole electric fields are generated in the space between the electrodes by the application of a direct-current (DC) voltage, U, and a radio-frequency (RF) driving voltage VAF,FCOSΩt, between the electrodes. The stability of the oscillation of the ions that have been trapped in these fields is dictated by the values “a” and “q” in expression (1) below that are given by the size of the apparatus (namely, inner radius “r0” of the ring electrode), the DC voltage, U, applied to each electrode, the amplitude, V, and the angular frequency, Ω, of the RF driving voltage, and the mass-to-charge ratio, “m/z” (kg/coulomb), of the ion.                               a          =                                                    8                ⁢                eU                                                              r                  0                  2                                ⁢                                  Ω                  2                                                      ·                          z              m                                      ,                  q          =                                                    4                ⁢                e                ⁢                                                                   ⁢                                  V                  RF                                                                              r                  0                  2                                ⁢                                  Ω                  2                                                      ·                          z              m                                                          (        1        )            
In the above expression, “z”, “m”, and “e” denote the valence number, mass, and elementary charge, respectively, of the ion. The stability region denoting the range of “a” and “q” in which stability of ion oscillation is given in the ion trap inter-electrode space is shown in FIG. 3.
Since only RF driving voltage VRFCOSΩt is applied to the ring electrode, all ions equivalent to the line of “a=0” in the stability region are usually oscillated in the space and trapped between the electrodes. At this time, the point of (0, q) on the stability region differs according to the particular mass-to-charge ratio “m/z” of the ion, and each ion is arranged “a”-axially between “q=0” and “q=0.908” on the line of “a=0” in order of the magnitude of the mass-to-charge ratio, subject to expression (1) above.
In the ion trap type mass spectrometer, therefore, all species of ions whose mass-to-charge ratios fall within a certain range are stably pre-trapped, at which time, the ions oscillate at a different frequency, depending on the “m/z” value. This characteristic is utilized for auxiliary AC electric fields of a specific frequency to be superimposed in the ion trap inter-electrode space, and only the ions that resonate with the auxiliary AC electric field therewith undergo mass separation.
Of all ions in the specimen, only those to undergo mass separation are sequentially scanned in terms of mass (mass scan analysis) to obtain a mass distribution chart (mass spectral chart) of all ingredients in the specimen. At this time, the quantity of ions which can be trapped in the ion trap inter-electrode space is realistically limited because increases in the quantity of ions trapped increase the effects of the space charge and thus reduce the analyzing performance of the apparatus.
Therefore, when the mass range (“m/z” range) of undesired ions, or ions not to be analyzed, is known or when the mass range (“m/z” range) of the necessary ions, or the ions to be analyzed, is known, all species of undesired ions can be ejected from the ion trap inter-electrode space before the ions in the specimen undergo mass spectral analysis.
Once all unnecessary ions have been ejected from the specimen, the number of necessary species of ions trapped in the ion trap inter-electrode space will correspondingly increase and thus analytical sensitivity will increase. Also, when only ions of a specific mass number (namely, parent ions) are trapped, dissociated, and undergo tandem mass spectral analysis (MS/MS analysis) to obtain the mass distribution of the dissociated ions, the quantity of parent ions trapped can be increased by ejecting all non-parent ions as undesired ion species. In addition, the creation of dissociated ions from non-parent ions can be avoided.
Since this MS/MS analytical method enables the acquisition of further detailed information on the molecular structure of specific ions, the MS/MS analytical function has come to be among the most important functional requirements of a mass spectrometer in recent years.
Various methods of eliminating all undesired ions whose “m/z” values fall within the required range have been developed up to now. For example, a method of ejecting such ions by applying wideband signals to the ion trap electrodes during the mass spectrographic scanning period is disclosed in U.S. Pat. No. 4,761,545.
Also, methods in which all undesired having their own oscillational frequencies falling outside the specified band are ejected by applying a frequency band-pass filter to noise waveforms are disclosed in U.S. Pat. No. 5,134,286 and Japanese Application Patent Laid-Open Publication No. Hei-7-509097.
In the above-mentioned examples, although the two methods differ in that whether they use a frequency band-pass filter, such a wideband auxiliary AC voltage as shown in expression (2) below, is applied to the ion trap inter-electrode space.                                           V            FNF                    =                                    ∑              i              n                        ⁢                                          v                i                            ⁢              sin              ⁢                                                           ⁢                              (                                                      ω                    i                                    +                                      ϕ                    i                                                  )                                                    ,                                            ω                              i                +                1                                      -                          ω              1                                =                      Δ            ⁢                                                   ⁢            ω                                              (        2        )            
Although these methods have heretofore been proposed for phase control between frequency components, since constant values are set for the amplitude, Vi, of each frequency component and the angular frequency division width, Δω, between frequency components, no control has been provided as to the wideband auxiliary AC voltage, VFFNF, or as to the amplitude, Vi, of each frequency component or the frequency division width, Δω, between frequency components according to the RF driving voltage value VRF.